For context, we demonstrate the results of two fracture propagation modeling cases with different structure of rock interfaces with respect to the horizontal wellbore. In both examples, one hydraulic fracture is initiated at the horizontal wellbore and propagates in vertical and horizontal directions. The rock properties and in-situ stresses are the same in different layers dividing by the prescribed interfaces for both presented examples. The interfaces are cohesionless but frictional planes of weakness.
Case of Symmetrical Interfaces with Respect to Wellbore
In the first example, horizontal interfaces are located symmetrically with respect to the horizontal wellbore. Hydraulic fracture initiated and propagates across these interfaces as well as along them in the horizontal direction, as shown in the FIG. 1. FIG. 1 shows a hydraulic fracture propagating from the horizontal wellbore in the case of symmetrical placement of horizontal interfaces with respect to wellbore.
Propagating of both vertical tips of hydraulic fracture across the interfaces is relatively slow because of continuous stops at each if these interfaces. At the same time, lateral tips of the hydraulic fracture propagate without interaction with interfaces (parallel to them). As a result, the length of hydraulic fracture appears to be much longer than its height (FIG. 2).
FIG. 2 shows an upper, lower, and lateral fracture tip propagation with time of fluid injection (upper graph), and corresponding pressure response at the fracture inlet (lower graph) for symmetrical placement of the interfaces.
Case of Asymmetrical Interfaces with Respect to Wellbore
In the second modeling case, cohesionless horizontal interfaces are positioned asymmetrically with respect to the wellbore. Number of interfaces below the wellbore is less than that above the wellbore (see FIG. 3). The pumping schedule, the spacing between the interfaces, and all other parameters of the rock and fracture remain the same, as in the first example. FIG. 3 shows hydraulic fracture propagating from the horizontal wellbore in the case of asymmetrical placement of horizontal interfaces with respect to wellbore.
Modeling shows that in this case after crossing two interfaces below the wellbore, the hydraulic fracture will be completely stopped at one of the upper interfaces while freely propagates downward (FIG. 4). FIG. 4 illustrates an upper, lower, and lateral fracture tip propagation with time of fluid injection (upper graph), and corresponding pressure response at the fracture inlet (lower graph) for asymmetrical placement of the interfaces.
These two examples indicate that the preliminary measurement of the weakness planes in rock and adequate modeling of fracture propagation in a layered formation are needed to identify fracture height containment in a layered rock adequately. And oppositely, missing the information about the heterogeneous profile of the rock strength in the vertical direction and prominent interfaces can result in wrong results in prediction of the fracture height containment conditioned by interaction of the hydraulic fracture with weakness planes.
Hydraulic fracturing used for the purpose of reservoir stimulation typically aims at propagating sufficiently long fractures in a reservoir. The fracture length can be as large as several hundred meters in horizontal direction. With such fracture extent the layered rock structure reveals severe heterogeneity vertically. Depending of the rock type, sedimentary laminations or beddings can have thickness in the range of millimeters to meters. Unequal variation of rock properties in vertical and horizontal directions results in noticeable restriction of the fracture height growth with respect to lateral fracture propagation. Since the beginning of fracturing era attention to the hydraulic fracture height containment was always recognized.
Subsurface three-dimensional propagation of hydraulic fractures (hereafter HF) typically implies simultaneous fracture growth in horizontal and vertical directions. Typical horizontal HF extent during field treatments varies from tens to hundreds meters along the intended formation layer. As opposed to that, vertical fracture extent appears much shorter in size because of large contrast of rock properties and tectonic stresses, as well as pre-existing horizontal bedding and lamination interfaces. There are several recognized mechanisms controlling the vertical HF growth (upward or downward) in geologic formations: (1) minimum horizontal stress variation as a function of depth (hereafter called “stress contrast” or “mechanism 1”), (2) elastic moduli contrast between adjacent and different lithological layers (hereafter called “elasticity contrast” or “mechanism 2”), and (3) weak mechanical interface between similar or different lithological layers (hereafter called “weak interface” or “mechanism 3”). A “weak mechanical interface” or “weak interface” or “plane of weakness” refers to any mechanical discontinuity that has low bonding strength (shear, tensile, stress intensity, friction) with respect to the strength of the rock matrix. A weak interface represents a potential barrier for fracture propagation as follows: when the HF reaches the weak interface, it creates a slip zone near the contact as shown by both analytical and numerical studies. Slip near the contact zone can arrest fracture propagation and lead to extensive fluid infiltration or even hydraulic opening of the interface by forming so called T-shape fractures. Such T-shape fractures have been repeatedly observed in various mineback observations in coal bed formations.
Nowadays, the “stress contrast” mechanism is the main used in most HF modeling codes to control vertical height growth, both for pseudo3D and planar3D models. The “elastic contrast” mechanism is usually not explicitly modeled in most HF modeling codes, but is in some way addressed by the “stress contrast” mechanism as vertical stress profile of minimum horizontal stress are often derived from a calibrated poroelastic model and overburden stress profile (isotropic and transverse isotropy can be treated) that depends on the elasticity of the formation. The “weak interface” mechanism has drawn less attention in the hydraulic fracturing community up to date, though it has been well recognized from field fracturing jobs and discussed in literature as far back as the 1980s. This lack of interest may have been caused by the lack of characterization of the location of the weak interfaces in deep formations and/or the lack of measurements of their mechanical properties (shear and tensile strength, fracture toughness, friction coefficient and permeability). At the same time the “weak interface” mechanism is one of the only of the above mechanisms that can completely stop the HF from further propagating upward or downward in formations. The main reasons for fracture tip termination at weak interfaces are the interface slippage, pressurization by penetrated fracturing fluid, or even mechanical opening of the interface. In contrast, the first two mechanisms may only temporarily stop the HF until the net pressure is increased in the HF up to a threshold level that will allow the HF to further propagate. The “weak interface” containment mechanism may be more important than “stress” or “elastic contrast” mechanisms and may be the reason why HF are often well contained in vertical extent despite apparent absence of any observed “stress” or “elastic contrast.” In any event, more effective methods for formation characterization, existing fracture influence on fracture development, and characterization of fracture generation are needed.